1. Field of the Invention
The present invention relates to a wavelength stabilized laser module and more particularly to the wavelength stabilized laser module capable of emitting laser light whose wavelength is stabilized with high accuracy and of being so configured as to be simple in structure and being made smaller in size.
The present application claims priority of Japanese Patent Application No.2000-067606 filed on Mar. 10, 2000, which is hereby incorporated by reference.
2. Description of the Related Art
Conventionally, a semiconductor laser device is used as an optical source for optical fiber communication. A single axial mode semiconductor laser such as a DFB (Distributed FeedBack laser) laser is employed, in particular, for optical fiber communication over distances of tens of kilometers or more in order to prevent an adverse effect on chromatic dispersion. However, though the DFB laser oscillates at a single wavelength, its oscillation wavelength is changed depending on a temperature of the semiconductor laser device and/or an injected current. Moreover, in the optical fiber communication system, since it is important to keep output strength of a semiconductor laser light source at a constant level, control is conventionally exercised so as to keep the temperature of the semiconductor laser device and the output strength of the semiconductor laser light source at the constant level. Basically, by keeping the temperature of the semiconductor laser device and the injected current at the constant level, a light output and oscillation wavelength of the semiconductor laser device can remain constant. However, if the semiconductor laser device is degraded due to longtime use, the injected current required for keeping the light output at the constant level increases, as a result, causing the oscillation wavelength to change. However, since changed amounts of the oscillation wavelength are slight, substantially no problem has occurred in the conventional optical fiber communication system.
In recent years, a DWDM (Dense Wavelength Division Multiplexing) method in which multiple pieces of light each having a different wavelength are multiplexed in one optical fiber becomes mainstream in the conventional optical fiber communication system and an interval among a plurality of the oscillation wavelengths used for the DWDM system becomes as narrow as 100 GHz or 50 GHz. In this case, a degree of wavelength stabilization required for the semiconductor laser device being used as the light source is, for example, xc2x150 pm, which means that conventional method in which temperatures of the semiconductor laser device and outputs of the semiconductor laser light are controlled so as to be kept at the constant level is not sufficient to obtain the required degree of wavelength stabilization. Moreover, even if the temperature of the semiconductor laser itself can be successfully controlled so as to remain constant, every time an ambient temperature surrounding the semiconductor laser device changes, the oscillation wavelength is also changed slightly and cases are increasing in which such a slight change in the oscillation wavelength causes a problem in the recent conventional optical fiber communication system.
In order to prevent such changes in the oscillation wavelength of the semiconductor laser light and to stabilize the oscillation wavelength, some conventional methods for stabilization have been proposed. A first example of the conventional wavelength stabilizing device (hereinafter being referred to as a first conventional example) is disclosed in, for example, Japanese Patent No. 2989775 (JP. Appln. Laid-open No. Hei 10-209546, in which, as shown in FIG. 19, a wavelength stabilizing device 128 is housed in a case mounted separately from a semiconductor laser module. A part of laser light is branched through a coupler 109 from an optical fiber transmission path 108 and introduced into the wavelength stabilizing device 128. In the wavelength stabilizing device 128, a filter 103 serving as a band pass filter is embedded, and an optical detector 111 used to detect light transmitted through the filter 103 and an optical detector 110 used to detect light reflected off the filter 103 are placed opposite to each other. FIG. 20 is a diagram showing optical current spectra to explain operations of the wavelength stabilizing device 128. As shown in FIG. 20, the transmitted light detected by the optical detector 111 and the reflected light detected by the optical detector 110 are in opposite phase relative to the oscillation wavelength of the semiconductor laser light. By calibrating the filter 103 and the optical detectors 110 and 111 so that a point of intersection of the reflected light and transmitted light, which is indicated by an arrow in FIG. 20, becomes a targeted wavelength for stabilization and by feeding back the transmitted light and reflected light to a temperature controlling unit (not shown) attached to the semiconductor laser device so that the transmitted light and reflected light become equal in light strength, the stabilization of oscillation wavelength of the semiconductor laser is achieved. Moreover, a slide adjusting unit 112 to set a reference wavelength to be used as a target for the stabilization is mounted on the filter 103.
The conventional wavelength stabilizing device 128 has problems in that, since it is basically housed in the case mounted separately from the semiconductor laser module, additional space for its installation is required, thus causing an increase in costs. Moreover, since a part of the signal light is branched by the coupler 109, the branched light is attenuated. Though the targeted reference wavelength can be set only by adjusting a position of the filter 103 using the slide adjusting unit 112, a specially-fabricated expensive filter is required which is so worked as to change its transmission characteristics in a direction of its plane by gradually changing internal thickness of the filter. Furthermore, since transmission characteristics of a filter, in general, are changed depending on a temperature of the filter itself, a separate process of adjusting the temperature of the filter 103 or a special electric circuit that can compensate for changes in transmission characteristics caused by the temperature is required.
A second example of the conventional wavelength stabilizing device adapted to prevent changes in an oscillation wavelength of a semiconductor laser light and to stabilize its wavelength is disclosed in Japanese Patent No. 2914748 (JP. Appln. Laid-open No. Hei 4-157780, which is shown in FIG. 21. A basic principle of stabilizing the wavelength in this wavelength stabilizing device is the same as provided in the first conventional example; that is, a part of signal light is branched and incident on a filter 103, and reflected light and transmitted light from the filter 103 are detected by the optical detectors 110 and 111 respectively, which are then fed back to the temperature controlling unit (not shown) of the semiconductor laser. The second conventional example differs from the first conventional example in that a frequency setting section 113 is provided, which is used to adjust an angle of the filter 103.
However, there is a problem in that, when the angle of the filter 103 is adjusted, since a direction of the reflected light from the filter 103 is also changed, a position of the optical detector 110 used to detect the reflected light has to be adjusted accordingly. In the second conventional example, though a method for adjusting a temperature of the filter 103, method for changing an electro-optic effect of the filter 103 or a like are also disclosed, it is actually difficult to put these methods to practical use.
A third example of the conventional wavelength stabilizing device adapted to prevent changes in an oscillation wavelength of a semiconductor laser and to stabilize its wavelength is disclosed in Japanese Patent Application Laid-open No. Hei 9-219554, which is shown in FIG. 22. This wavelength stabilizing device differs from the first and second conventional wavelength stabilizing devices in that light emitted from a semiconductor laser module 101 is branched by a beam splitter 115 having no dependence on a wavelength of light and each piece of the branched light is received by optical detectors 110 and 111 respectively and that a filter 117 whose transmittance becomes low with decreasing wavelength of light is mounted in front of the optical detector 110 and a filter 116 whose transmittance becomes high with increasing wavelength of light is mounted in front of the optical detector 111. As a result, as in the cases of the first and second conventional examples, by adjusting a balance of signals fed from the two optical detectors 110 and 111, the wavelength of light emitted from the semiconductor laser module 101 can be stabilized. In this method, though units adapted to adjust angles of the filters 116 and 117 for matching of the wavelength are required, since transmitted light is incident on both the filters 116 and 117, there is no need for adjusting positions of the optical detectors 110 and 111 when the angles of the filters 116 and 117 are adjusted.
However, it is actually impossible to house so many components including the beam splitter 115, the filter 117 and the detector 110 to be mounted in a vertical direction to an optical axis of the laser oscillation light in a tightly-spaced case of the semiconductor laser module 101. Moreover, it is also difficult to obtain a sufficient quantity of light transmitted through the filter 117 to stabilize the wavelength of the laser light unless an optical system is used which is adapted to cause light emitted from the semiconductor laser module 101 to converge to be parallel luminous flux, with aid of a lens or a like. This is because, since the semiconductor laser module 101 emits light at a relatively large radiation angle, as a distance between a surface from which the semiconductor laser is radiated and optical detectors 110 and 111 becomes far, strength of detection of the light is rapidly decreased. As shown in FIG. 23, if a light receiving area of an optical detector 104 is made large in order to increase its detecting sensitivity, an area of the filter 103 on which light is incident also increases and a big difference in an angle of incidence on the filter 103 occurs due to positional reasons. That is, a difference in the angle of incidence between rays A and B becomes large. Since the wavelength filter used in the example has a property in which its transmission characteristic depends greatly on the incident angle of light, regardless of whether the filter is of a multilayer type or of an etalon type, as is the case of the multilayer type shown in FIG. 24, there is a big difference in transmission characteristics between the rays A and B each having a big difference in the angle of incidence of light on the filter 103 and, as a result, in some cases, the dependence on wavelength in light receiving strength of the entire emitted laser becomes small or disappears. To avoid this problem, it is necessary to cause light emitted from the semiconductor laser module 101 to converge to be parallel luminous flux. However, in this case, component counts increase and adjustment of parts including the lens used to cause the light to converge to the parallel luminous flux, filters 116 and 117, beam splitter 115, optical detectors 110 and 111 or the like becomes complicated, thus causing an increase in manufacturing costs.
A fourth example of the conventional wavelength stabilizing device adapted to prevent changes in an oscillation wavelength of a semiconductor laser and to stabilize its wavelength is disclosed in Japanese Patent Application Laid-open No. Hei 10-79723, which is shown in FIG. 25. In the disclosed wavelength stabilizing device, in order to obtain a signal whose transmittance becomes high with increasing wavelength and a signal whose transmittance becomes low with decreasing wavelength, light emitted from a semiconductor laser module 101 is adjusted by using a lens 102 so as to be emitted at a specified diffusion angle and the diffused light is then incident on a tilt filter 103a and the light transmitted through the tilt filter 103a is detected by an optical detector 104 having two light receiving planes 105 and 106. Since light to be incident on the light receiving planes 105 and light to be incident on the light receiving planes 106 differ from each other in incident angles on the tilt filter 103a, with only the one tilt filter 103a, plural different transmission characteristics can be provided.
However, in the fourth conventional example, precise adjustment of optical systems employed in the wavelength stabilizing device is required and characteristics of light transmitted through the tilt filter 103a occurring when the light is incident on light receiving planes 105 and 106 are changed intricately due to changes in a diffusion angle of semiconductor laser light caused by an even slight change of a position of the lens 102, a change in an angle of the tilt filter 103a, a change in a position of the optical detector 104 or a like. That is, to independently control the tilt filter 103a transmitting characteristic affecting the incidence of the light on the light receiving planes 105 and 106 and to stabilize the wavelength so as to have a specified wavelength, highly accurate placement of each of parts making up the wavelength stabilizing device is required. For example, since it is impossible to stabilize the wavelength only by adjusting the angle of the tilt filter 103a, a big problem occurs when the wavelength stabilizing device is actually fabricated.
A fifth example of the conventional wavelength stabilizing device adapted to prevent changes in an oscillation wavelength of a semiconductor laser and to stabilize its wavelength is disclosed in Japanese Patent Application Laid-open No. Hei 9-121070, which is shown in FIG. 26. In the disclosed wavelength stabilizing device, backward emitted light from a semiconductor laser module 101 is branched by a beam splitter 115 and one branched light beam is incident directly on an optical detector 110 without being incident through a filter 103, which is used for detection of optical strength of the light, while another branched light beam is incident on an optical detector 111 through the filter 103, which is used for detection of a wavelength of the light. In this case, by controlling the optical detector 110 used to detect light not passing through the filter 103 so that its optical current is made constant, an output from the semiconductor laser module 101 can be controlled so as to remain constant. In the case of the light transmitted through the filter 103, as is understood from dependence of transmittance of the filter 103 on wavelengths, that is, dependence of an optical current xe2x80x9cIxe2x80x9d of the optical detector 111 on the wavelength as shown in FIG. 5, by stabilizing the output current used as a signal for the detection of the wavelength so as to become a constant value xe2x80x9cI0xe2x80x9d, the output of the light and its oscillation wavelength can be simultaneously controlled.
However, it is actually difficult to embed optical systems required for stabilizing a wavelength of laser light including the beam splitter 115 together with optical systems required for the semiconductor laser module 101 into a tightly-spaced case. Moreover, as in the case of the third conventional example, unless the optical system that can cause light emitted from the semiconductor laser module 101 to converge to be a parallel luminous flux is used, sufficient light transmitted through the filter 103 cannot be obtained. As a result, it is necessary to use a lens as an additional optical system, thus further increasing component counts. Placement and arrangement of each of components including lens, filter, and beam splitter are made complicated, causing an increase in manufacturing costs.
As described above, in the conventional wavelength stabilizing device for the semiconductor laser device, component counts are large and big space are required and therefore it is difficult to house all these components into the limited and tightly-spaced case for the semiconductor laser device module. Nevertheless, in the conventional wavelength stabilizing device, as in the case of the third example, unless optical systems by which semiconductor laser light is converged to be parallel luminous flux, filter emitted light that can satisfactorily achieve the stabilization of the wavelength can not be obtained. If the lens is incorporated in the optical system, component counts increase and further the adjustment of such components as the lens, filter, beam splitter, detectors or the like becomes complicated, leading to increased manufacturing costs.
As described above, in the conventional wavelength stabilizing devices of the semiconductor laser, component counts are very large which causes an increase in the required space and therefore it is not only difficult to house such conventional wavelength stabilizing devices into the case used for the conventional semiconductor laser module but also difficult to set the reference wavelength to be used as a target for the stabilization.
In view of the above, it is an object of the present invention to provide a wavelength stabilized laser module having small component counts and being compact enough to be housed in such a tightly-spaced case for a semiconductor laser module as has been conventionally used and being capable of setting a reference wavelength very easily with high accuracy and being manufactured at low costs.
According to a first aspect of the present invention, there is provided a wavelength stabilized laser module including:
a semiconductor laser;
a temperature calibrating unit to calibrate a temperature of the semiconductor laser;
a converting unit to convert light emitted from the semiconductor laser to parallel luminous flux;
a first photoelectric converting unit to receive a part of the parallel luminous flux and to convert it to an electric signal;
a filter to receive a part of the parallel luminous flux and to continuously change its transmittance depending on wavelengths of the parallel luminous flux;
a second photoelectric converting unit to receive light transmitted through the filter and to convert it to an electric signal; and
wherein a control signal, to be used for stabilization, obtained by computations of the electric signal fed from the first photoelectric converting unit and the second photoelectric converting unit, is fed back to the semiconductor laser and/or the temperature calibrating unit so that the semiconductor laser is able to stably emit laser light having a reference wavelength to be used as a target for stabilization of wavelength.
With the above configuration, since one part of the luminous flux which is emitted from the semiconductor laser and is converted to the parallel luminous flux is received by the first photoelectric converting unit and another part of the luminous flux, after having been transmitted through the filter adapted to continuously change transmittance depending on wavelengths, is received by the second photoelectric converting unit, the first photoelectric converting unit can take out optical currents that change depending on an optical output from the semiconductor laser and the second converting unit can take out optical currents that change depending on an optical output fed from the semiconductor laser and depending on wavelengths. Therefore, by performing computations of the two optical currents taken from the wavelength stabilized laser module, each of a current value that changes depending on the optical output and a current value that changes depending on wavelengths can be independently obtained. Since, in the filter, a relationship between the wavelength and transmittance is clear, by comparing the current value that changes depending on wavelengths presently detected with the current value that is output according to the reference wavelength to be used as the target for stabilization of wavelengths, a shift between the wavelength of light presently emitted from the semiconductor laser and the reference wavelength can be calculated. Moreover, since a wavelength of semiconductor laser light generally changes depending on an injected current and a temperature of the semiconductor device, by feeding back the control signal to cause the shifted amount to be zero to the injected current adjusting device and the temperature calibrating unit of the semiconductor laser, variations in wavelengths of the semiconductor laser light can be reduced and laser light with the reference wavelength being stabilized with high accuracy can be emitted. Variations in optical outputs from the semiconductor laser have been detected by the first photoelectric converting unit, by feeding back the output shifted signal to the semiconductor laser, optical outputs from the semiconductor laser are stabilized with high accuracy. Furthermore, since parts such the beam splitter as has been used conventionally are not employed to obtain a signal that changes depending on the wavelength and a signal that does not change depending on the wavelength, component counts can be reduced and the space usage efficiency is made higher, thus enabling configurations of the wavelength stabilized laser module to be compact enough to be housed in such the tightly-spaced case as has been used for the conventional semiconductor laser module and, since the adjustment and assembly at a time of manufacturing of the module are made simpler due to reduced component counts, manufacturing costs are greatly reduced.
In the foregoing, a preferable mode is one wherein the first photoelectric converting unit and second photoelectric converting unit are so configured as to receive backward emitted light from the semiconductor laser.
With the above configuration, since all amounts of forward emitted light from the semiconductor laser can be utilized fully for optical communication, unlike the conventional examples, no loss of transmitted light power caused by branching of a part of the laser light from the optical transmission path that has been required for detection of wavelengths occurs.
Also, a preferable mode is one wherein the converting unit to convert light emitted from the semiconductor laser to parallel luminous flux is a lens and wherein one part of the single parallel luminous flux transmitted through the lens is incident on the first photoelectric converting unit and another part of the parallel flux is incident on the filter.
With the above configuration, by converting diffused light emitted from the semiconductor laser to the parallel luminous flux using the lens and by inserting the filter into a part of cross faces of the single parallel luminous flux transmitted through the lens so that one part of the parallel luminous flux is incident on the filter and another part of the parallel luminous flux is incident on the first photoelectric converting unit, adverse effects on transmission characteristics caused by differences in incident angles of light into the filter by places of light incidence can be prevented, thus enabling the stabilization of wavelengths with high accuracy and further inhibiting a decrease in light receiving strength, caused by the diffusion of laser light, in the first and second photoelectric converting units. Moreover, there is no need for mounting a lens separately for each of the first and second photoelectric converting units, thus serving to reduce component counts and to achieve the compact wavelength stabilized laser module.
Also, a preferable mode is one wherein a degree of parallelization of the parallel luminous flux is within xc2x12xc2x0.
With the above configuration, adverse effects on the transmission characteristics caused by differences in incident angles by portions of the filter can be minimized, thus achieving the highly accurate stabilization of wavelengths.
Also, a preferable mode is one wherein the filter has a transmission characteristic in which transmittance of the filter becomes high or low monotonically depending on wavelengths within a band of wavelengths containing the reference wavelength.
With the above configuration, by selecting the filter whose gradient of the transmission spectrum monotonically becomes high or low, the second photoelectric converting unit can immediately detect the wavelength of the laser light that changes on a long wavelength side or a short wavelength side relative to the reference wavelength, as a change on a bright side or a dark side of the light transmitted through the filter.
Also, a preferable mode is one wherein the filter is able to change, by adjusting an angle of incidence, a gradient of changes in transmittance which changes depending on wavelengths.
With the above configuration, if the gradient in changes in transmittance that changes depending on the wavelength can be varied by adjusting the angle of incidence, by making the gradient sharp, detecting sensitivity to changes in wavelengths can be improved and the stabilization of wavelengths with high accuracy is made possible, while, by making the gradient gentle, a band width of wavelength in which changes are detected can be expanded.
Also, a preferable mode is one wherein the filter has a unimodal transmission characteristic in which transmittance of the filter becomes maximum and minimum in a band of wavelengths not containing the reference wavelength.
With the above configuration, if the reference wavelength is in a maximum transmission band or in a minimum transmission band of the transmission characteristic, the sensitivity to changes in the wavelengths is greatly lowered. When the transmission characteristic is unimodal, in a wide band of wavelengths except the maximum transmission band or the minimum transmission band being limited bands, detection of wavelengths with high sensitivity is made possible.
Also, a preferable mode is one wherein the filter is a multilayer filter made up of dielectric multilayers formed on a transparent substrate.
With the above configuration, when the multilayer filter is used, a thickness of a glass substrate can be set arbitrarily, thus enabling the substrate to be thin and achieving the compact wavelength stabilized laser module.
Also, a preferable mode is one wherein the filter is an etalon-type filter exhibiting a transmittance period in which transmittance of the filter becomes maximum and minimum repeatedly at an interval of a specified wavelength.
With the above configuration, since the etalon-type filter has a plurality of maximum points and minimum points within a band of wavelengths of light that can be emitted from the semiconductor laser, each of the reference wavelengths can be set to be placed on the gradient of the spectrum between each of the maximum points and each of the minimum points and stabilization of the plurality of the reference wavelengths can be achieved in the multiple optical system using, as the light source, the wavelength tunable semiconductor laser in the single wavelength stabilized laser module.
Also, a preferable mode is one wherein the semiconductor laser is a wavelength tunable semiconductor laser that is able to emit light having the plurality of wavelengths which change depending on temperatures and the interval of wavelengths in the transmittance period of the etalon-type filter is set by an equation:
D=(1xe2x88x92Tetalon/TLD)xc3x97D0xe2x80x83xe2x80x83(1)
where xe2x80x9cDxe2x80x9d represents the wavelength interval in the transmittance period of the etalon-type filter, xe2x80x9cD0xe2x80x9d represents an interval of the plurality of wavelengths of light emitted from the semiconductor laser, xe2x80x9cTetalonxe2x80x9d represents an amount of a change in a central wavelength occurring when a temperature of the etalon-type filter changes by 1xc2x0 C. and xe2x80x9cTLDxe2x80x9d represents an amount of a change in an oscillation wavelength occurring when a temperature of the semiconductor laser changes by 1xc2x0 C., however, the central wavelength represents one wavelength that causes the transmittance to be maximum.
With the above configuration, when the wavelength interval of the transmittance period of the etalon-type filter obtained by the above equation is set by using the semiconductor laser that can change the wavelength depending on temperatures, the plurality of reference wavelengths can be set to be placed on the gradient of the spectrum between the maximum point and the minimum point of the transmittance period, thus serving to the stabilization of each of wavelengths of light emitted from the semiconductor laser in the single wavelength stabilized laser module.
Also, a preferable mode is one wherein the filter is made up of a transparent material having reflectivity being higher than that of silica glass.
Also, a preferable mode is one wherein the transparent material is a Si (Silicon)-based material.
With the above configurations, by using the transparent base material having high reflectivity as the material for the etalon-type filter or multilayer filter rather than the silica glass that has been conventionally used, a thickness of the filter can be made thin and the space required for mounting the wavelength stabilized laser module can be further reduced. The Si-based material is transparent and has reflectivity being higher than that of the silica glass and is the relatively low-cost material widely used in the semiconductor fields and therefore it is best suitable as the material for the filter of the present invention.
Also, a preferable mode is one wherein the filter is fixed to the second photoelectric converting unit.
Also, a preferable mode is one wherein the filter is formed on a light receiving surface of the second photoelectric converting unit by a coating method.
With the above configurations, by mounting the filter and the photoelectric converting unit in an integral manner, more compact configurations of the wavelength stabilized laser module can be achieved, when compared with the case in which these components are mounted in a separate manner.
Also, a preferable mode is one wherein the first and second photoelectric units are placed in parallel on a holding substrate and make up an array-shaped optical detector.
With the above configuration, since complicated adjustment of angle is not required, by using these converting units as part of the optical detector with these converting units mounted in parallel, component counts and man-hours needed to assemble can be reduced, thus serving to lower manufacturing costs.
Also, a preferable mode is one wherein a light receiving surface of the first photoelectric converting unit is placed in a tilt manner relative to an optical axis of incident light.
With the above configuration, feedback light by reflection which is reflected off the light receiving surface of the first photoelectric converting unit toward the semiconductor laser can be removed, thus reducing changes in the oscillation characteristics of the semiconductor laser caused by the feedback light.
Also, a preferable mode is one wherein the semiconductor laser has a configuration of a device integrated with an electroabsorption-type semiconductor optical modulator.
With the above configuration, since the semiconductor laser is integral with the electroabsorption-type semiconductor optical modulator, when compared with the case in which each of the DFB laser and the outside modulator is mounted as a separate module, it is possible to make the entire optical transmission system compact.
Also, a preferable mode is one wherein the temperature calibrating unit is a Peltier device.
With the above configuration, since the Peltier device can set a temperature accurately within an arbitrary range of temperatures under electronic control and the Peltier device can be configured to be thin, it can be housed in the case as an integral part of the module substrate.
Furthermore, a preferable mode is one that wherein includes an optical fiber used as a device through which laser light is output and a single case housing, at least, the semiconductor laser, the temperature calibrating unit, the converting unit for conversion to parallel luminous flux, the filter and the first and second photoelectric converting units.
With the above configuration, since component counts of the wavelength stabilizing device are small and its adjustment is made easy, it can be housed in such the small-sized case as has been conventionally used for housing the semiconductor laser module with no wavelength stabilizing device.